1. Field of the Invention
The present invention relates to a PTC (Positive Temperature Coefficient) thermistor, and more particularly to a surface-mount PTC thermistor mounted to a PCB (Printed Circuit Board) for protecting circuits.
2. Description of the Related Art
It is well known that many conductive materials changes in their specific resistances according to the change of temperature. The element whose resistance varies according to temperature is commonly called ‘a thermistor’, which is generally classified into an NTC (Negative Temperature Coefficient) element showing a decrease of resistance with temperature increasing, and a PTC (Positive Temperature Coefficient) element showing an increase of resistance with temperature increasing.
The PTC element shows a low resistance at a low temperature, namely at a room temperature, so that current may pass through it. However, if the operating circumstance of the element is heated or the temperature of the element rises due to an over current, the resistance of the PTC element increases as much as 1,000 to 10,000 times of its normal resistance. Due to such properties, the PTC element is usually mounted on a PCB (Printed Circuit Board) for controlling an over current.
The PCB has many elements and components on its surface, so each component is restricted by size. Thus, there have been suggested various types of PTC elements to overcome such kind of restrictions. Most commonly, the PTC element is sandwiched between a pair of laminated electrodes.
FIG. 1 shows a PTC thermistor disclosed in U.S. Pat. No. 5,907,272. Referring to FIG. 1, a first electrode 250 and a second electrode 260 are laminated respectively on upper and lower surfaces of a PTC element 210 so that PTC element 210 is sandwiched between the electrodes. In addition, the PTC element and the first and second electrodes are surrounded by an insulating layer 280. And gaps 290 and 300 are respectively formed to expose electrodes. After the gaps are formed, one of the first and second electrodes 250 and 260 laminated on upper or lower surfaces of the PTC element is extended to the opposite surface so that the PTC thermistor can be mounted on a PCB surface. To realize it, a terminal 320 which electrically connects the gap 300 with the first electrode 250 is formed at a portion of the lower surface, while a terminal 310 which covers upper, side and lower surfaces of the insulating layer 280 and electrically connects the gap 290 with the second electrode 260 is formed at the other portion of the lower surface.
However, the above method of electrically connecting one electrode at one surface of the PTC thermistor to the other surface is apt to cause the so-called Tombstone phenomenon. When a thermistor is mounted on the PCB, the thermistor of which the terminals 310 and 320 are coated with solder cream in advance is arranged on an electrode pad of the PCB and then heated in the reflow machine. At this time, however, the heat applied to the thermistor expands the PTC element 210 and the terminals 310 and 320. Since the PTC element and the terminals have different thermal expansion coefficients and the above-described thermistor has an asymmetric configuration, thermal stress distribution is not uniform in right and left portions of the thermistor, so the thermistor is inclined on the surface of the PCB. This considerably deteriorates physical and electrical reliability of the soldering.
In addition, since the current flow mainly exists between the upper and lower surfaces in the prior art, a plurality of PTC thermistors, each having one layer, should be laminated in multi layers in order to lower the resistance of the PTC thermistor in a limited space of the PCB.